Normalizing circuit



Nqv. 8, 1938. D. v. SINNINGER NORMALIZING CIRCUIT Filed June 1'7, 195?Patented Nov. 8, 1938 UNITED STATES PATENT OFFICE 2,135,852 N ORMALIZING CIRCUIT Dwight V. Sinninger,

Chicago, Ill., assignor to Johnson Laboratories, Inc., Chicago, Ill., acorporation of Illinois Application June 17,

1 Claim.

narrow band of frequencies lying on either side of its nominal resonantfrequency.

several serious problems which must be solved in the design of asatisfactory multi-stage amplifier. One of these difficulties is theinherent grid-plate capacitance of the vacuum tube.

grid-plate capacitance may be reduced substantially by the use of enoughto contribute to the dency to oscillate of a multi-stage vacuum-tubeamplifier, thus seriously limiting the useful gain or amplificationwhich may be realized.

An object of this invention is to provide simple and inexpensive meansfor substantially eliminating the undesirable effects of vacuum-tubeoutput capacitance in high-gain, multi-stage amplifiers.

amplifier.

An additional object of the invention is to provide means forsubstantially increasing the stable amplification which may be realizedin a multi stage vacuum-tube amplifier.

1937, Serial No. 148,668

Although the arrangements of the invention are applicable to varioustypes of high-frequency vacuum-tube amplifiers, they are especiallyuseheterodyne radio receiver vacuum tube I might Dotted capacifixedcapacitor 50.

Although coupling device 3 is shown to consist of two inductivelycoupled inductors l2 and I3 which are tuned by adjustable capacitors I land I5, re-

suitable coupling tween vacuum tubes I and 2. For example, a couplingdevice employing three sented by dotted capacitor l6.

Although the major portion of the high-frequency current which flows ininductor I2 passes through tuning capacitor [4, some current flows alsothrough by-pass capacitor 5 and output capacitance 4 of vacuum tube 1,which are effectively in series across tuned circuit l2--l4. Assumingthat capacitor l were momentarily shortcircuited and capacitor Iremoved, it is evident that a portion of the circulating current oftuned circuit 8-9 would flow through capacitor 6 which is effectively inseries with the output capacitance of vacuum tube 2. Such a common pathfor portions of the high-frequency currents flowing in the platecircuits of two successive amplifying vacuum tubes introduces seriousfeedback difficulties, with resultant instability and an unsymmetricalselectivity characteristic for the amplifier as a whole.

The invention provides means whereby the coupling which is common tocircuits l2-l4 and 8-9 may be balanced out. The operation of thearrangement of Fig. 1 may be better understood by reference to Fig. 2,in which like parts are designated by like reference numerals.Capacitors 5, I, 9 and I0 form the four arms of a capacitance bridge,across one diagonal of which is connected inductor 8 and across theother diagonal of which is connected by-pass capacitor 6. Resistor II,in shunt with capacitor l0, has no appreciable effect upon the operationof the bridge, but is necessary to provide a direct-current path to theplate of vacuum tube 2. A choke coil may be substituted for resistor H.

The value of capacitor 5 depends upon the characteristics of vacuum tube2 and thus is not readily adjustable. Capacitors 9 and I0 in series arein shunt with inductor 8, so that their values cannot be changed withoutaffecting the resonant frequency of the output circuit of vacuum tube 2.In practice, capacitor is fixed at a suitable value and capacitor 9 isadjustable to-provide a means for aligning the circuit 8-9 at thedesired frequency. Capacitor I may be given any desired value in orderthat the bridge may have the proper adjustment to eliminate the effectsof undesired couplings between circuits l2-l4 and 8--9, and will requireonly a slight readjustment of capacitor 1.

In practice, it has been found that the degree of mis-adjustmentintroduced by vacuum tube replacement or by alignment of tuned circuit89 is not sufficient to introduce serious feedback difiiculties in anotherwise properly adjusted system. It is within the scope of theinvention, however, to provide means for re-adjusting the bridge ifnecessary, as for example by making capacitor 1 adjustable.

If the grid-plate capacitance l6 of vacuum tube 2 is assumed to be zero,the only coupling between the output circuit of vacuum tube 4 and theoutput circuit of vacuum tube 2 is that due to capacitor 6. Thiscoupling may be balanced out by so choosing the value of capacitor 1that the ratio of capacitors l0 and I is equal to the ratio ofcapacitors 9 and 5. This is the condition which would exist wereadditional means, not shown, to be employed to balance out the effect ofgrid-plate capacitance 16. When the bridge is balanced in this case,therefore, there is no common coupling or tendency toward feedback dueto by-pass capacitor 6, and the system is stable.

In most cases, however, the grid-plate capacitance I6 of vacuum tube 2is not neutralized, and thus provides a coupling between circuits 12-and 8-9 which either aids or opposes the coupling due to capacitor 6,depending upon the polarity of inductor l3 with respect to that tance l6aids that of capacitor 6, the bridge must be unbalanced in the propersense and degree to nullify the sum of the two couplings. This may beaccomplished by so choosing capacitor 1 that the ratio of capacitors l0and 1 is enough smaller than the ratio of capacitors 9 and 5 tocompensate for the combined undesired couplings. The constants givenbelow for an illustrative embodiment of the invention apply to a case ofthis type.

If the coupling due to grid-plate capacitance I6 opposes but is lessthan the coupling due to capacitor 6, the bridge must be unbalanced tocompensate for the difference between the two couplings. This may beaccomplished by so choosing capacitor 1 that the ratio of capacitors l0and I is sufiiciently greater than the ratio of capacitors 9 and 5 tocompensate for the coupling which remains. If the bridge constants aresuitably chosen to meet this condition, the system will be free frominstability due to coupling between circuits |2--l4 and 8-9.

By way of example of one successful embodiment of the arrangements shownin Fig. 1 of the drawing, the following constants are given for a systemdesigned to operate at a frequency of 460 kilocycles:

Vacuum tubes 1 and 2 Capacitors 4 and 5 Capacitor 6 Capacitor 7Capacitors 9 and 14 Type 6K7.

12 micromicrofarads.

0.05 microfarad.

0.005 microfarad.

104 micromicrofarads (approximate setting). Capacitor 10 0.01microfarad. Resistor 11 800 ohms.

Inductors 8 and 12 It will be understood that these are approximatevalues and that the scope of the invention is in no way limited toembodiments employing the same or even closely similar values. Adifierent operating frequency or the use of different types of vacuumtubes may require changes in some of the constants, and such changes areboth contemplated and within the scope of the invention. 7

Having thus described my invention, what I claim is:

In a high-frequency amplifier, a first vacuum tube having a plate and aplate circuit, said plate circuit including a parallel tuned circuit anda source of plate potential shunted by a capacitor; a second vacuum tubehaving a plate and a plate circuit, said last-mentioned plate circuitincluding a parallel tuned circuit comprising an inductor shunted by anadjustable capacitor and a fixed capacitor in series, said fixedcapacitor being shunted by a resistor; a connection from the junction ofsaid adjustable capacitor and said fixed capacitor to the positiveterminal of said source of plate potential; and a capacitor connectedbetween ground and the junction of said inductor and said resistor.

DWIGHT V. SINNINGER.

1.0 millihenry.

